Wireless communications have become popular as a reliable and cost-effective alternative to hard-wire communication transfer. Along with infrared and microwave radio, spread spectrum technology is employed in many wireless communication systems. Spread spectrum is a type of modulation that scatters data transmission across an available frequency band in a pseudo-random pattern. Thus, spread spectrum employs a signal resistant to noise, interference, and eavesdropping. Additionally, spread spectrum is an efficient way to use radio waves as multiple users may share radio frequencies at the same time without interfering with each other.
In a spread spectrum signal, a modulated carrier is further modulated by a spreading code that spreads the signal over a larger bandwidth. This may be accomplished through a direct sequence system, frequency hopping, and time hopping. Direct sequence and frequency hopping systems utilize pseudo random sequences to spread a signal across the entire bandwidth. Direct sequence utilizes a pseudo random sequence to encode transmitted digital data to randomize the output spectrum wherein frequency hopping utilizes a pseudo random sequence to randomly alter the center frequency of the carrier from a set of frequencies.
The employment of pseudo random sequences makes spread spectrum appear wide band and noise-like. This characteristic is advantageous because it provides intercept prevention. Spread spectrum signals are difficult to detect on narrow band equipment because the signal's energy is spread over a bandwidth many times greater than the information bandwidth. The utilization of a pseudo random sequence is further advantageous because a transmitter and receiver may each execute the same pseudo random sequence to aid in the synchronization of multiple users.
A drawback associated with conventional schemes for generating pseudo random sequences known to the art is the ability of an eavesdropper to intercept the random sequence. For example, a potential eavesdropper may effectively learn a hop set created from the pseudo random sequence for a frequency hopping spread spectrum system by measuring the time between control point beacons on separate channels. As a result, an eavesdropper may have the capability of intercepting spread spectrum signals upon determining the pseudo random sequence.
Another disadvantage to utilization of pseudo random sequences is the requirement of additional hardware or processor operation to generate the pseudo random sequences. Hardware random number generators require the use of a resistor and detection equipment designed to measure the noise present across the resistor. Other types of hardware random number generators may include shift registers and logic circuitry. Thus, hardware random number generators require additional components that add cost and occupy chip real estate on a spread spectrum radio. Software random number generators utilize a program of instructions executed by a microprocessor. The microprocessor performs multiple iterations of an algorithm to generate a pseudo random sequence. This requires additional processor action which increases power consumption. Spread spectrum technology is frequently employed in remote, battery-powered devices in which space and power consumption is restricted. Consequently, a method and apparatus for generating a pseudo random number for a spread spectrum system that prevents interception and provides real estate and power consumption efficiency is necessary.